Polyamide and polyolefin alloys, especially polyamide and polypropylene alloys are polymeric materials which are in general obtained by blending a polyamide component, consisting of one or more polyamides, with a polyolefin component consisting of one or more polyolefins such as, for example, polypropylene, and an agent capable of making the two polymeric components compatible, or else by blending a polyamide component, consisting of one or more polyamides, with a modified polyolefin component consisting of one or more polyolefins, especially polypropylene, at least one of which has been modified to improve the compatibility of the polyolefin component with regard to the polyamide component. The said alloys offer an advantageous compromise between the different properties of each of the polymeric components.
It is known that polyamides are characterized by good heat stability but that their dimensional stability is affected by a relatively high water uptake, whereas polypropylene, which does not absorb water, has a dimensional stability which is not affected by the relative humidity of the surrounding air, but has a moderate heat stability and a dimensional stability which is lower than that of dry polyamide.
A polyamide and polypropylene alloy, on the other hand, will exhibit a dimensional stability intermediate between that of polypropylene and that of dry polyamide, a water uptake close to that of polypropylene and consequently a dimensional stability independent of the relative humidity of the surrounding air and also a satisfactory heat stability.
Polyamide and polyolefin alloys can be employed in particular in the motor vehicle industry sector for the production of decorative components such as wheel embellishers and body wings, or in other fields and, in particular, for the production of garden furniture.
For use in the production of decorative components for motor vehicles, a polymer alloy must have, inter alia, a good heat resistance at temperatures of the order of 180.degree. C., which correspond to the temperature for baking the paints employed for motor vehicle body components, so that the said components made of polymer alloy can be inserted into a painting line for this same purpose as metal components, as well as a satisfactory paintability.
It is known to produce polyamide and polypropylene alloys which have the abovementioned heat stability for the production of decorative components for a motor vehicle but, on the other hand, the paintability of such components still presents a problem because of an insufficient polarity of the surface of the alloy component, which is reflected in mediocre adhesiveness between the paint coating and the said surface. An adhesiveness of this kind does not meet a motor vehicle industry specification and it is necessary to resort to a surface treatment of the said alloy component before it is painted, in order to improve its paintability.
It has been found that the surface treatment of an object made from a polyamide and polyolefin alloy by employing a cold plasma, that is to say a reactive gas flow resulting from the action of an electrical discharge on a gas, and an oxygen source constitutes a technically effective and economically viable solution for improving the paintability of the said object and, in particular, for making the adhesiveness between the paint film and the surface of the object conform to a motor vehicle industry specification.